Process and device for carrying out a venous plethysmography using compression

ABSTRACT

Method and apparatus for venous compression plethysmography with a strain-gauge or by direct circumferential length measurement, the calibration of the measurement equipment being done automotively.

BACKGROUND OF THE INVENTION

The present invention relates to a method and an apparatus for venouscompression plethysmography.

Venous compression plethysmography is a procedure which has been knownfor some time now and which is used for determining microvascularparameters in the extremities, such as the venous capacity, the venouselasticity, the venous outflow rate, the arterial blood flow and thecapillary filtration rate. In general, venous compressionplethysmography allows qualitative and quantitative statements to bemade concerning the state and function of the microvascular circulationin an extremity of a patient.

Venous compression plethysmography can be carried out in a very widevariety of ways, for example as water plethysmography, airplethysmography, impedance plethysmography, capacitance plethysmography,induction plethysmography or strain-gauge plethysmography. Theseprocedures make use of different physical phenomena for determining thestate of the blood vessels in a body part. The present invention relatesto strain-gauge plethysmography and to compression plethysmography bymeans of direct induction-based measurement of circumferential change.

In strain-gauge plethysmography, an expandable strain-gauge is laidaround the body part which is to be examined, for example an arm or aleg. The venous return of the blood is then obstructed in this body partusing an inflatable cuff arranged nearer to the heart. The bloodcongestion leads to a change in the circumference of the body part inquestion, which in turn leads to expansion of the strain-gauge. From theexpansion of the strain-gauge, which depends on the pressure applied tothe cuff, it is possible to draw conclusions regarding characteristicsor changes in the blood vessels. This evaluation of the expansion as afunction of the induced congestion is based on known procedures.

In strain-gauge plethysmography today, it is normal to use astrain-gauge consisting of an expandable silicone tubing filled withmercury. In the event of expansion of the body part around which thestrain-gauge is laid, the silicone tubing expands and deforms themercury column located therein. As a result, the electrical resistanceof the mercury column changes. This change in resistance is measured andfrom this it is then possible to draw conclusions regarding theexpansion of the silicone tubing and, consequently, of the body part.This conclusion necessarily assumes knowledge of the relationshipbetween the change in resistance and the strain-gauge expansion.Apparatuses for strain-gauge plethysmography must therefore becalibrated prior to their use. This calibration must be carried outvirtually before each single examination of a body part, since the ratioof the resistance of the mercury column to the expansion of thestrain-gauge depends on a large number of parameters, such as theambient temperature and the patient's body temperature, the initialstress of the silicone tubing and the circumference of the body partexamined.

The calibration is carried out by expanding the strain-gauge in adefined manner, while it is fastened in the examination position, and bymeasuring the change in resistance which occurs. In this case, severalsuccessive defined expansions of the strain-gauge are normally carriedout, and the associated changes in resistance measured. A conversionfactor is then calculated from this, assuming a linear relationship orother relationship of the measurement parameters. For calibrationpurposes, a calibration apparatus is arranged on the strain-gauge, andin conventional equipment this calibration apparatus generally consistsof a knurled screw for expanding the strain-gauge. During calibration,the operator, for example a doctor or a nurse, then successively adjuststhe length of the strain-gauge manually.

With this manual changing of the expansion, the operator is continuouslyexerting disturbing forces on the mostly very light measuring system, sothat, for an exact measurement, it is necessary to wait for therelaxation of the system again and again. The relaxation times of thesystem can in these cases be very long and very different. An additionalexpansion of the silicone tubing, for example, can relax relativelyquickly, whilst an impression in the tissue of the body part to beexamined requires very much more time to return to the initial state.This leads to considerable time losses in the examination. Moreover, inthe case of manual contact with the measurement device which bears onthe body part, there is the risk of the strain-gauge being shifted onthe patient's skin. This can lead to errors in the calibration and,thus, to inaccuracies in the evaluation of the measured values. Aparticular disadvantage is that it very much depends on the skill of theoperator as to whether the measurement can be carried out quickly andreliably. The measurement thus loses some of its reliability as well asits reproducibility. For the measurement error caused by the operator toremain unchanged during a series of measurements, it would in fact benecessary for the same operator to carry out all the measurements. Thisis of course not really possible in prolonged series of tests andresearch projects.

SUMMARY OF THE INVENTION

The object of the invention is therefore to make available a method andan apparatus for strain-gauge plethysmography, which method is quick andeasy to use but at the same time reliably provides accurate measuredvalues.

According to one aspect of the invention there is provided a method forvenous compression plethysmography, in which an extremity is surroundedby a cuff, whose internal diameter I can be varied, in such a way thatan obstruction of the blood outflow can be generated in those veins ofthe extremity which are situated remote from the heart in isolation tothe cuff, where a strain-gauge is arranged on the extremity in order toencircle the latter, at a point remote from the hear in relation to thecuff, in such a way that a tissue distension of the extremity, occurringas a result of an obstruction of the blood outflow, causes an expansionΔD of the strain-gauge, where the expansion ΔD of the strain gauge, as afunction of a measurement of the change ΔI of the internal diameter I ofthe cuff, is detected by determining the change ΔM of a measurementparameter M, and where the relationship between the strain-gaugeexpansion ΔD and the measurement parameter change ΔM is determined bycalibration with the aid of a calibration apparatus connected to thestrain-gauge, by determining at least one measurement parameter changeΔM₁₂ for a defined expansion ΔD₁₂, characterized in that the definedexpansion ΔD for the calibration is generated by an adjustment mechanismin the calibration apparatus without an operator touching thestrain-gauge or the calibration apparatus.

According to a further aspect of the invention, there is provided anapparatus for venous compression plethsymography with a cuff, whoseinternal diameter I can be varied, and which is suitable for encirclingan extremity, with a measurement device arranged distally thereof,characterized in that the measurement device consists of a first areawhich is laid around the extremity and has an essentially low-expansion,dimensionally non-stable force transmission element which is guidedround the circumference of an essentially band-like support belt, of asecond area which is in operational communication with the two ends ofthe low-expansion, dimensionally non-stable force transmission elementin such a way that a circumferential linear change of the extremity, bymeans of the low-expansion, dimensionally non-stable force transmissionelement, is detected by a measurement apparatus, where one end of thelow-expansion, dimensionally non-stable force transmission element is incontact with the measurement apparatus, and the other end of thelow-expansion, dimensionally non-stable force transmission element issecured on an adjustable bearing arrangement, or in that the measurementdevice consists of a first area which is laid around the extremity andhas an essentially expandable, dimensionally non-stable forcetransmission element which is guided round the circumference of anessentially band-like support belt, of a second area which is inoperational communication with the two ends of the expandable,dimensionally non-stable force transmission element in such a way that acircumferential linear change of the extremity, by means of theexpandable, dimensionally non-stable force transmission element, isdetected by a measurement apparatus, where one end of the expandable,dimensionally non-stable force transmission element is in contact withthe measurement apparatus, and the other end of the expandable,dimensionally non-stable force transmission element is secured on anadjustable bearing arrangement.

In the context of the present invention, a method and an apparatus aremade available for venous compression plethysmography using astrain-gauge, where the strain-gauge is calibrated without being touchedby an operator. The calibration is preferably done automotively using anelectric motor, a pneumatic mechanism or a spring mechanism, which motoror mechanism provides for the adjustment of a knurled screw or otheradvancing device, so that the desired defined expansion can be set. Useof a microprocessor for controlling the calibration device isparticularly preferred. In this way, the calibration of the expansionmeasurement can be undertaken without direct contact between theoperator and the strain-gauge or calibration device.

BRIEF DESCRIPTION OF THE DRAWINGS

The apparatus according to the invention and the method according to theinvention are described below with reference to the figures, where:

FIG. 1 shows the principle of strain-gauge plethysmography,

FIG. 2 shows the apparatus according to the invention for strain-gaugeplethysmography.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows an apparatus for venous compression plethysmography with acuff 2, which encircles a body part 1, for example a leg, and whoseinternal diameter I can be varied using a pump 3. The apparatus furthercomprises a strain-gauge 4 which likewise encircles the body part 1. Thestrain-gauge 4 consists of a tubular flexible receiving material,preferably a silicone tubing, and of an expandable and electricallyconducting material located therein, preferably mercury or amercury-containing mixture. An electrical contact is in each casearranged at both ends of the strain-gauge 4. These contacts areconnected via leads 5 to recording device 6, which is suitable formeasuring the electrical resistance in the substance or a complementaryvariable such as the voltage drop or the current flow. The signals fromthe recording device 6 and the pump 3 are then fed to an evaluation unit7, where the measurement results are evaluated as a function of ameasure of the change in the internal diameter of the cuff 2, generallythe pressure exerted on the cuff 2 by the pump 3.

Instead of the electrical resistance, any other desired measurementparameter M can be used as well, for example the number of interferencelines of superimposing light beams, whose change ΔM is a measure of theexpansion of the strain-gauge 4. The strain gauge can then be of acorrespondingly different design, and does not therefore need to consistof a receiving material and of an electrically conducting substancearranged therein.

The strain-gauge 4 must be movable in the circumferential direction onthe body part 1 in order to be able to expand along its entire length inthe event of a change in the circumference of the body part 1.Otherwise, the strain-gauge 4 could remain stuck at one particular pointof the body part 1, so that it would be locally overexpanded there,while not being expanded at all at other points. The consequence of thisis unreliable measurements. In general, therefore, a sliding band isplaced under the strain-gauge 4 to permit a movement of the strain-gauge4 along the circumference of the body part 1. This can be achieved bythe strain-gauge 4 sliding on the sliding band, or by the sliding bandsliding on the skin, or by both effects. It is also conceivable,however, for the strain-gauge 4 itself to be designed, for examplecoated, such that it slides on the skin.

A calibration device is arranged on the strain gauge 4. In the contextof the present invention, the calibration device is operated free ofcontact, that is to say without an operator touching it. This can bedone, for example, with the aid of an electric motor or a pneumaticmechanism or else a spring mechanism in the calibration device. Thecalibration is preferably controlled by a microprocessor.

FIG. 2 shows a particularly preferred embodiment of the apparatusaccording to the invention. This apparatus has a calibration device 10with at least the following elements: A linear drive 12 which iscontrolled without contact, for example electrically or pneumatically,with a push rod 18 which has a fastening point 19, and a tightening band23 which consists of a first part which, at fastening point 19, isconnected to the push rod 18 of the linear drive 12, and of a secondpart which is connected to the strain gauge 14 at a fastening point 20,the first part being guided as a loop through a bracket in the secondpart in such a way that the length of the tightening band 23 between thefastening points 19 and 20 can be varied. In this way, the measurementapparatus can be used on body parts of different circumference. Theother end of the strain-gauge 14 is connected to the calibration device10 at clamp device 13. The tightening band 23 can be guided via adeflector device 24 in order to ensure a correct travel. The end 25 ofthe tightening band 23 can be fixed on its opposite-running part, forexample with a Velcro closure. The calibration device 10 moreovercomprises a length measurement device 21 which can encircle the bodypart 1, but which can also consist only of a relatively small section.The length measurement device is arranged in such a way that the lengthof the strain-gauge 14 can be determined. The calibration device 10preferably has a fastening point 22 for fastening a sliding band 15encircling the body part 1, and, again preferably, it has a temperaturesensor 16 for measurement of the surface temperature of the body part 1.The arrangement of a further temperature sensor (not shown here) for theambient temperature is particularly preferred. If technically possible,a single temperature sensor can of course be used for determining theskin temperature and the ambient temperature. It is also possible for asingle temperature sensor to measure a combined temperature of skin andambient temperature if this temperature is suitable for correcting themeasured values.

The apparatus shown in FIG. 1 is operated as follows, in accordance withthe method according to the invention. A cuff 2 is placed around a bodypart 1 which is to be examined. This body part can be a relatively largeextremity, such as a leg or an arm, but smaller extremities, such asfingers or toes, may also be examined. When correspondingly pressurizedvia the pump 3, this cuff 2 generates a blockage of the return flow ofvenous blood in the distally situated body part 1. A strain-gauge 4 isthen arranged on the same body part, at a point more distant from theheart. When the body part 1 changes circumference as a result of theblood outflow obstruction which has been generated, the strain-gauge 4also expands correspondingly. Thus, for example, the substance in thestrain-gauge 4 changes its electrical resistance. This change inresistance is measured by a measuring unit 6 which is connected via lead5 to both ends of the strain-gauge 4. At a defined pressure on the cuff2, which is a measure of its internal diameter I or of the change ΔI inthis internal diameter, in order to take the change ΔR in the resistanceR of the substance and from this draw conclusions on an expansion ΔD ofthe strain-gauge 4, the apparatus must first be calibrated. Thecalibration is done by setting a defined expansion ΔD₁₂ with the aid ofthe calibration device, driven for example by an electric motor, and bymeasuring the associated value of the resistance change ΔR₁₂. This stepcan be repeated several times, depending on the requirements regardingmeasurement accuracy. The calibration is preferably carried out byevaluating two resistance changes ΔR₁₂ and ΔR₂₃ for two successivleydefined expansions ΔD₁₂ and ΔD₂₃. However, a calibration by means of atleast double determination of the resistance change ΔR₁₂ for the samedefined expansion ΔD₁₂ is preferred, in which case the expansion,between two determinations of the resistance change, is taken back tothe initial measure.

When using the apparatus according to FIG. 2, at the start of themeasurement the strain-gauge 14 on sliding band 15 is placed around thebody part 1. To do this, the first part of the tightening band 23 isguided through the bracket of the second part of the tightening band 23and then fastened, for example using a Velcro closure, on theopposite-running part of the tightening band 23. The tightening band 23is then set so that the expandable strain-gauge 14 surrounding the bodypart 1 has a defined predetermined length L₀. This is possible becausethe strain-gauge 14 does not encircle the entire body part 1, but isinterrupted by the calibration device 10. The setting of a definedeffective initial length L₀ of the strain-gauge 14 for the measurementis necessary because the force needed for a relative expansion ΔDdepends on the absolute length or initial expansion of the strain-gauge14. Effective length is here understood as the length of thestrain-gauge 14 lying on the body part 1. The length gauge 21 which isarranged on the calibration device 10 and at least partially enclosesthe body part 1 permits determination of the effective length of thestrain-gauge 14. By adjusting the length of the tightening band 23, thedesired length L₀ of the strain gauge 14 can be set. With the aid of thelength gauge 21, the unexpanded circumference of the body part 1 in themeasurement plane can also be determined. If the length gauge 21encloses the body part 1 completely, the circumference of the body partis measured directly; if the gauge 21 only partially encloses the bodypart 1, the desired size must be determined with the help of the knownlength, for example, of the sliding band 15.

During the measurement, the skin temperature of the body part 1 and theambient temperature should be observed where possible. A change in themeasured temperature by only a few degrees can already lead to anexpansion of the strain-gauge 14, which leads to a significantmeasurement error. For this reason, a temperature sensor 16 for thesurface temperature of the body part 1 is preferably provided. Since themeasured temperature need not necessarily correspond with thetemperature of the skin of the body part 1, a further temperature sensorcan also be provided for the ambient temperature. A single measurementsensor can also be provided for both temperatures or for a combinedtemperature. According to the signals from the temperature sensors, thelength of the strain-gauge 14 can then be readjusted, or the measurementotherwise corrected, or an error signal can be output which leads to thetermination of the measurement.

The method according to the invention and the corresponding apparatusafford the requirements necessary for simply and reliably determiningthe absolute values of microvascular parameters and also of theirperiodic fluctuations.

It is a further object of the invention to make available an apparatusfor compression plethysmography which effects a direct circumferentiallinear change and does not have the disadvantages of the prior art.

This object is achieved by the apparatus according to Patent claim 1.

In the context of the present invention, an apparatus is made availablefor venous compression plethysmography using a direct circumferentiallinear change, where the inductive displacement measuring device iscalibrated without being touched by an operator. The calibration ispreferably done automotively using an electric motor, a pneumaticmechanism or a spring mechanism, which motor or mechanism provides forthe adjustment of a knurled screw or other advancing device, so that thedesired defined expansion can be set. Use of a microprocessor forcontrolling the calibration device is particularly preferred. In thisway, the calibration of the circumferential linear change can beundertaken without direct contact between the operator and thedisplacement measuring device or calibration device.

The apparatus according to the invention and the method according to theinvention are described below with reference to the figures, where

FIG. 3 shows the apparatus for compression plethysmography according tothe invention,

FIG. 4 shows an exploded view of the apparatus according to theinvention,

FIG. 5 shows a perspective view of the supporting belt according to theinvention.

FIG. 1 shows an apparatus for venous compression plethysmography with acuff 2, which encircles a body part 1, for example a leg, and whoseinternal diameter I can be varied using a pump 3. The apparatus furthercomprises a measurement device 4 which likewise encircles the body part1. The measurement device 4 consists of a band-shaped support belt, anadjustable bearing arrangement 11, a displacement measuring device 10and a low-expansion, dimensionally non-stable force transmission element7.

The values obtained via the displacement measuring device are connectedvia leads 5 to recording device 6. The signals from the recording device6 and the pump 3 are then fed to an evaluation unit 7, where themeasurement results are evaluated as a function of a measure of thechange in the internal diameter of the cuff 2, generally the pressureexerted on the cuff 2 by the pump 3.

A calibration device is arranged on the measurement device 4. Thecalibration device is operated free of contact, that is to say withoutan operator touching it. This can be done, for example, with the aid ofan electric motor or a pneumatic mechanism or else a spring mechanism inthe calibration device. The calibration is preferably controlled by amicroprocessor.

FIG. 3 shows a particularly preferred embodiment of the apparatusaccording to the invention. This apparatus has an adjustable bearingarrangement with at least the following elements: An actuator 10controlled without contact, for example electrically or pneumatically,with a push rod 13, a gear stage, which consists of two spur wheels, anda coupling piece 21, on which the low-expansion, dimensionallynon-stable force transmission element can be coupled.

FIG. 4 shows an exploded view of the individual structural units of themeasurement device 4 according to the invention. The displacementmeasuring device 10 of the apparatus according to the invention consistsof a cylinder 19 which is moved in a bore 18. By means of the movement,an induction voltage is induced, so that the distance travelled is inrelation to the induced voltage.

Another design of the displacement measuring device of the apparatusaccording to the invention is also possible, for example piezoelectronicsensors which can deliver a corresponding signal, or optic displacementmeasuring devices which can measure distances, for example byincremental transmitters.

Another embodiment integrates the bearing arrangement and thedisplacement measuring device in such a way that, for calibration, onlyone signal has to be called up, namely the induction voltage in relationto the spindle height. This can be done, for example, in apiezoelectronic actuator.

The bearing arrangement 11 according to the invention has an electricactuator 12 to which a gear stage 13 is connected downstream. The gearstage 13 has a spur wheel 14 a on the front of the electric actuator 12and a spur wheel 14 b on the front of a spindle rod 15, the hub of thespur wheel 14 b being of an open design, and the low-expansion,dimensionally non-stable force transmission element 30 being connectedreleasably to the spindle rod. The actuator 12 drives the gear stage 13,and the spindle rod 15 executes a linear movement through an internalthread in spur wheel 14 b. This linear movement is able to tighten orloosen the low-expansion, dimensionally non-stable force transmissionelement by means of coupling piece 21, so that controlled readjustmentof the force transmission element can take place.

The apparatus shown in FIG. 1 is operated as follows. A cuff 2 is placedaround a body part 1 which is to be examined. This body part can be arelatively large extremity, such as a leg or an arm, but smallerextremities, such as fingers or toes, may also be examined. Whencorrespondingly pressurized via the pump 3, this cuff 2 generates ablockage of the return flow of venous blood in the distally situatedbody part 1. A measurement device 4 is then arranged on the same bodypart, at a point more distant from the heart. When the body part 1changes circumference as a result of the blood outflow obstruction whichhas been generated, the support belt is expanded, which results in anexcursion of the force transmission element in the displacementmeasuring device, as a result of which a voltage is induced. This changein voltage is measured by a measuring unit 6 which is connected via lead5 to both ends of the strain-gauge 4. At a defined pressure on the cuff2, which is a measure of its internal diameter I or of the change ΔI inthis internal diameter, in order to take the change ΔV in thedisplacement measuring device and from this draw conclusions on a linearchange ΔL of the force transmission element, the apparatus must first becalibrated. The calibration is done by setting a defined expansion ΔL₁₂with the aid of the calibration device, driven for example by anelectric motor, and by measuring the associated value of the resistancechange ΔV₁₂. This step can be repeated several times, depending on therequirements regarding measurement accuracy. The calibration ispreferably carried out by evaluating two voltage changes ΔV₁₂ and ΔV₂₃for two successively defined expansions ΔL₁₂ and ΔL₂₃. However, acalibration by means of at least double determination of the voltagechange ΔV₁₂ for the same defined expansion ΔL₁₂ is preferred, in whichcase the expansion, between two determinations of the voltage change, istaken back to the initial measure.

When using the apparatus according to FIG. 4, at the start of themeasurement the support belt is placed around the body part 1. Thelength of the support belt can be varied, as a function of thecircumference of the extremity, by means of individual elements beingconnected in modular fashion to each other. This is preferably done bysnap connections which ensure both a secure fit and reliablereleasability. After the measurement device has been placed in thedesired manner on the extremity, the calibration is performed.

FIG. 5 shows the band shaped support belt 9 according to the inventionwhich serves as a bearing on the skin of the extremity 1 and is designedsuch that it affords a reliable fit on the surface of the skin,essentially by adhesion, so that it is possible to prevent themeasurement device from slipping. The support belt 9 preferably has ameandering cross-section which extends in the longitudinal direction. Bymeans of this meandering configuration, the support belt 9 can beexpanded in the longitudinal direction upon tensile loading, and socarry along the guide devices 20 located on the top surface. The guidingof the force transmission elements 30 is thus at all times reliable andessentially free from friction. The support belt 9 moreover has modularindividual elements which can be connected to one another by areleasable snap connection in such a way that the length of the supportbelt can be adjusted as desired, in order to take account of theconditions of different extremities.

For the safe and reliable guiding of the force transmission member 30elements, the modular individual elements have devices which arepreferably of circular design.

The low-expansion, dimensionally non-stable force transmission member 30is preferably a yarn made of a polyester material which is furtherdistinguished by a smooth and thus low-friction surface. Other materialsare also possible for the force transmission member 30, for examplepolyamide yarns or carbon fibres.

The apparatus according to the invention affords the requirementsnecessary for simply and reliably determining the absolute values ofmicrovascular parameters and also of their periodic fluctuations.

What is claimed is:
 1. Method for venous compression plethysmography, inwhich an extremity is surrounded by a cuff, whose internal diameter canbe varied in such a way that an obstruction of the blood outflow can begenerated in those veins of the extremity which are situated remote fromthe heart in relation to the cuff, said method comprising: arranging astrain-gauge on the extremity in order to encircle the latter at a pointremote from the heart in relation to the cuff whereby a tissuedistension of the extremity occurring as a result of an obstruction ofthe blood outflow causes an expansion of the strain-gauge, detecting theexpansion of the strain-gauge as a function of a measurement of thechange of the internal diameter of the cuff by determining the change ofa measurement parameter, and determining a relationship between thestrain-gauge expansion and the measurement parameter change bycalibrating with a calibration apparatus connected to the strain-gauge,by determining at least one measurement parameter change for a definedexpansion, wherein the defined expansion for the calibration isgenerated by a hands-free adjustment mechanism in the calibrationapparatus, said adjustment mechanism adapted for hands-free operationwithout an operator touching the strain gauge or the calibrationapparatus.
 2. Method according to claim 1, characterized in that thereceiving material is a silicone tubing.
 3. Method according to claim 2,characterized in that the substance is expandable and electricallyconducting.
 4. Method according to claim 1, characterized in that thedefined expansion for the calibration is generated with the aid of oneof an electric motor a pneumatic mechanism, or spring mechanism in thecalibration apparatus.
 5. Method according to claim 1, characterized inthat the calibration is controlled by a microprocessor.
 6. Methodaccording to claim 1, characterized in that the calibration is effectedby evaluation of two measurement parameter changes for two successivedefined expansions.
 7. Method according to claim 1, characterized inthat the calibration is effected by at least twice determining themeasurement parameter change for the same defined expansion, in whichcase the expansion between two determinations of the measurementparameter change is taken back to the initial measure.
 8. Methodaccording to claim 1, characterized in that the substance is expandableand electrically conducting.
 9. Apparatus for venous compressionplethysmography with a cuff, whose internal diameter can be varied, andwhich is suitable for encircling a body part, with a strain-gauge whichis suitable for encircling the body part, and with a calibrationapparatus which is connected to the strain-gauge and permits a definedexpansion of the gauge, characterized in that the calibration apparatushas an adjustment mechanism adapted to expand the gauge by the definedexpansion, said adjustment mechanism adapted for hands-free operation.10. Apparatus according to claim 9, wherein the strain-gauge consists ofa receiving material and of an electrically conducting substance locatedtherein, and has contacts for measuring the electrical resistance in thesubstance.
 11. Apparatus according to claim 9, characterized in that thereceiving material is a silicone tubing.
 12. Apparatus according toclaim 11, characterized in that the substance includes mercury. 13.Apparatus according to claim 9, characterized in that the calibrationapparatus contains one of an electric motor and a pneumatic mechanismfor generating the defined expansion.
 14. Apparatus according to claim9, characterized by a microprocessor for controlling the calibration.15. Apparatus according to claim 9, characterized by said calibrationapparatus having: a contactlessly controlled linear drive with a pushrod which has a fastening point, a tightening band which consists of afirst part which, at one fastening point, is connected to the push rodof the linear drive, and of a second part connected to the strain-gaugeat a second fastening point, the first part being guided as a loopthrough a bracket in the second part whereby the length of thetightening band between the fastening points can be varied, and a lengthmeasurement device adapted to encircle the body part thereon whereby theeffective length of the strain-gauge and the circumference of the bodypart can be determined in a measurement plane.
 16. Apparatus accordingto claim 9, characterized by a fastening point (22) for fastening asliding band (15) encircling the body part (1).
 17. Apparatus accordingto claim 9, characterized by a temperature sensor for measurement of atleast one of the surface temperature and the ambient temperature of thebody part.
 18. Apparatus according to claim 9, characterized in that thesubstance includes mercury.
 19. Method for venous compressionplethysmography, in which an extremity is surrounded by a cuff, whoseinternal diameter can be varied in such a way that an obstruction of theblood outflow can be generated in those veins of the extremity which aresituated remote from the heart in relation to the cuff, said methodcomprising: arranging a strain-gauge on the extremity in order toencircle the latter at a point remote from the heart in relation to thecuff whereby a tissue distension of the extremity occurring as a resultof an obstruction of the blood outflow causes an expansion of thestrain-gauge, detecting the expansion of the strain-gauge as a functionof a measurement of the change of the internal diameter of the cuff bydetermining the change of a measurement parameter, and determining arelationship between the strain-gauge expansion and the measurementparameter change by calibrating with a calibration apparatus connectedto the strain-gauge, by determining at least one measurement parameterchange for a defined expansion, wherein the defined expansion for thecalibration is generated by a hands-free adjustment mechanism in thecalibration apparatus, said adjustment mechanism adapted for hands-freeoperation without an operator touching the strain gauge or thecalibration apparatus, wherein the electrical resistance in a substanceis used as said measurement parameter, where the strain-gauge consistsof a receiving material and of the substance located therein, and hascontacts for measuring the electrical resistance in the substance.